This invention relates generally to the field of electron beam computed tomography. More particularly, the present invention relates to methods for increasing the resolution of electron beam computed tomography scanners.
Electron beam computed tomography (EBCT) is a non-invasive imaging technique for capturing images of human organs. To capture images by using an EBCT scanner, a focused electron beam is directed onto tungsten targets. The focusing is done with the help of a focusing system. Due to the impinging of the electron beam, X-rays are produced at the tungsten targets. These X-rays are directed at semi-circular detectors. The X-rays pass through the object, which is to be imaged, and are collected by the detectors.
The resolution of an image obtained from an EBCT scanner depends on the width of the detectors, the collimation of the electron beam, and the width of the electron beam focus spot on the targets. A non-optimal beam spot width results in lack of sharpness or blurring at the edges of the images.
One method of measuring the width of the focal spot in an EBCT scanner is by passing the electron beam over a set of ‘w-wires’. This method is described in U.S. Pat. No. 5,224,137, titled “Tuning the Scanning Electron Beam Computed Tomography Scanner”, filed on May 23, 1991, and assigned to Imatron Inc. The w-wires are placed on a non-imaging target. A current that depends on the width of the focal spot is generated in the w-wires as the electron beam passes over them. This current is measured and used to quantify the width of the focal spot. The current supplied to the focusing coil is adjusted to obtain an optimal beam spot width for the non-imaging target. The current is then modified to derive the optimal beam spot width for the imaging targets by using theoretical calculations.
Other methods of finding the optimal current for the focusing coil use specialized phantoms for calculating the beam spot width. Further, these methods use non-imaging targets to find the beam spot width. This means that the beam spot may have unequal widths at different points on the surface of the imaging targets, reducing the resolution of the images captured. Finally, these methods are complicated and require a substantial amount of time to calculate the beam spot width.